go to chapters:
1. Overview
2. Study Area Description
3. Geomorphology and Sediment Cones
4. Environmental Resources
5.Problems and Opportunities
6. Recommendations
7. References
8. Appendices
Chapter 6. RECOMMENDATIONS
6.1 Recommendations
To have a meaningful impact over much of the study area, a systematic watershed approach is needed. With the reach serving as an international boundary, this would necessarily involve coordination and cooperation between the two nations to be most effective, as well as with the various regulatory and operating agencies. The primary ingredient for affecting significant environmentally beneficial change is effectively managing the water resource. Essential to this is a better understanding of the existing regime, coupled with predictive modeling to evaluate alternative scenarios to inform water managers of the most efficient usage of a scarce resource. The first step should be a meaningful water budget to quantify anticipated water volume, as well as identify/quantify depletions. Volume determination would aid in the evaluation of the reach’s response to variations in timing, magnitude, and duration; while depleting elements could be evaluated for modification/ enhancement.
A less encompassing, but potentially more workable, approach could entail selection of some “pilot” or “demonstration” sites for promoting environmental recovery. With appropriate planning, adaptive management and monitoring, the lessons learned at these sites could potentially be applied more broadly and perhaps more economically, throughout the study area. In executing this approach, a first-cut screening of the study reach would be useful – categorizing subreaches by similarities, such as biologic functions exhibited or desired as well as by geomorphic aspects. In selecting pilot project sites, consideration of the origin of adjacent tributaries could foreseeably be a primary ‘screening’ criterion. For example, incorporation of sediment retention basins on nearby tributaries to limit sediment supply to the mainstem Rio Grande within a demo site subreach could be complicated by the coordination with Mexico if the tributary comes in from that side of the river.
Finally, the ‘cluster’ of sediment cones noted in Chapter 3 (Figure 5) deserves some discussion. It would be expected that the apparently dramatic transverse elevation features visible in the area of the cluster would cause the river profile to be significantly less uniform within this region of the study area. This, in turn, could be exacerbating the overall sedimentation problems typically seen throughout the study area, assuming it is not a primary influence. It would follow that in formulating alternatives, this area should be considered for channel improvements, since mechanically modifying the channel geometry holds potential for yielding beneficial results. (return to table of contents)
6.2 Qualitative “Demonstration Site” Recommendations
Introduction: During the course of this study, three “pilot” sites were suggested for potential restoration, to facilitate adaptive management and to provide opportunities to learn lessons. The three sites suggested for demonstration projects were:
- Rancho Consuela
- Arroyo Ocho Cinco
- Cecil’s Pond
The following narrative will describe possible engineering features that could be used to achieve ecosystem restoration objectives associated with hydraulic and sedimentation behavior within the context of one of these sites, Cecil’s Pond. It is important to note that these are only developed conceptually, from very limited knowledge of the site conditions – more rigorous analysis, not to mention actual design, would be necessary to achieve success. None-the-less, it is anticipated that this exercise will prove useful to the study sponsor, by illustrating the types of engineered facilities that can be incorporated to meet environmental objectives.
Cecil’s Pond Background: Cecil’s Pond is the site of a mitigation project undertaken by the IBWC in the mid-80s for the Boundary Preservation Project that apparently was not maintained and currently has little habitat value remaining. (The remnants of this feature are visible on page 22 of the Map Book, between “sediment cones” 35 and 36.) The original project consisted of a rectangular pond, presumably excavated mechanically, used to supply drip irrigation to 1,022 cottonwood and willow trees planted in augered holes. The duration of irrigation apparently lasted for approximately 40 days, though a follow-up investigation recommended 75 days of irrigation (Anderson and Ohmart, 1986).
Current Conditions: Based on the 2004 aerial IR photography available for this study, as compared to the project sketch from the 1986 report cited above, the Rio Grande channel has migrated laterally away from the rectangular pond. The 1986 sketch shows the river channel parallel to the upstream-most half of the southern long edge of the pond, separated by approximately one channel-width of (cleared) overbank. The 2004 photography shows the channel has migrated away from its previous alignment along this edge of the pond, with a tortuous series of meanders approaching the upstream edge of the pond from the south (up-valley direction) and the downstream-most half of the pond edge now in closer proximity to the river channel. The river channel in this vicinity is reportedly narrow, incised, and heavily vegetated by exotics. The planform shape of the visible surface water of the pond is irregular and bounded by vegetation, suggesting asymmetric sediment deposition within the pond. This deposition could presumably be the result of high (overbanking) river flows, eolian (windborne) transport, or both. There is also visible surface water to the east, between the pond and County Highway 170. There are no significant arroyos flowing toward the pond, indicating it is wetted primarily by groundwater. (return to table of contents)
Potential Feature Array: Cecil’s Pond appears to be located on the periphery of the alluvial fan formed by the upstream left-bank arroyo (FID 35). If the restoration objectives included reducing sediment deposition in the mainstem river channel adjacent to this pilot site, a detention structure on the upstream arroyo would be worth considering. However, this reach is reportedly incised. Assuming the restoration objectives include raising the local groundwater table (e.g., to increase the pond surface area, depth, etc.), a low-head grade-control feature constructed in the river could have enough local influence to achieve this objective. This type of feature would effectively raise the channel bed for some distance upstream of itself, which would increase the width/depth ratio of the river, and increase the frequency of overbank flows (due to decreased channel capacity). It could also add some hydraulic diversity, with a pool/riffle effect. Careful design would be required for such a structure to reduce fish migration issues if this is a concern, in addition to assuring survivability during high flows. Alternatively, a portion of the overly-high overbank could be mechanically lowered by excavation, enhancing the hydrologic connectivity of the river with the floodplain. Either of these approaches could be combined with modification of the channel geometry to achieve desirable aquatic hydrodynamic conditions (e.g., wide, shallow channel form, increased sinuosity, etc.). If freshwater flow-through circulation was desired within the pond (e.g., to increase dissolved oxygen content, reduce temperature), a small distribution channel and return could be created to divert a portion of river flow. This would presumably require some ‘hardening’ to preclude capture by, or abandonment of, the active river channel. A ‘backwater channel’ could also be created in the vicinity within the overbank to create low-velocity aquatic habitat, incorporating bankline plantings to moderate water temperatures and enhance stability. Creation of a low-velocity deposition zone in the overbank upstream of the pond could help reduce deposition within the pond during high-flows, thereby reducing maintenance. This could be achieved by planting a dense ‘screen’ of, for example, willow cuttings which would increase local hydraulic roughness, reducing velocity and sediment transport capacity. Modern planting techniques reportedly have the potential to achieve much higher survival results than those used at the site originally.
Conclusion: The preceding conceptual approaches are offered to illustrate an array of features which could be selected from to help achieve desired biological functions at the site. The combination of prudent data collection, careful design and implementation, and watchful monitoring would yield insight for application to other sites within the study area. (return to table of contents)
